740 PATTERNS AND PROBLEMS OF DEVELOPMENT 



region and the entoderm, perhaps also the mesenchyme, is more gradual and reaches 

 full development at a later stage in Arbacia than in Strongylocentrotus. In any case, 

 a reinvestigation of differential susceptibility and differential dye reduction in Arbacia 

 is necessary for further light on these points. 



Attention must also be called to the possibility, noted elsewhere (Child, 1936a, 

 p. 449), that the dye -reduction picture exaggerates more or less the change in con- 

 dition in the basal region preceding gastrulation. The greater thickness of the wall 

 in the basal region of the late blastula may bring about a more rapid oxygen de- 

 crease, and consequently an earlier reduction in the cells of that region, than in the 

 thinner ectoderm. If that occurs, the differential reduction picture does not repre- 

 sent correctly the difference in physiological condition of ectoderm and entoderm 

 mesenchyme. However, this factor of thickness of wall and volume of cells does not 

 account wholly, if at all, for the apparent change in condition in the basal region. It 

 was noted in the text that reduction at all levels is most rapid at the inner ends or 

 surfaces of the cells, that is, the parts bounding the blastocoel. As immigration of 

 mesenchyme occurs, the cells which lie in the blastocoel reduce more rapidly than the 

 inner surfaces of any other cells of the blastula, and even before invagination the 

 inner surface of the entoderm reduces more rapidly than the inner surfaces of ecto- 

 derm cells, except perhaps those in the apical region. Since this was not the case in 

 earlier stages, a real change in physiological condition of mesenchyme and entoderm 

 has evidently occurred. 



It is also an interesting question whether this change is induced in the entoderm 

 by the mesenchyme or is independent of it. The evidence of a similar, though appar- 

 ently less extreme, change in the basal region of the Patina blastula in which no pri- 

 mary mesenchyme is formed shows that in that form it appears in the entoderm with- 

 out induction. 



Even though evidence of a change in condition in the basal region seems to be con- 

 clusive, it may perhaps still be questioned whether this region actually attains before 

 gastrulation a higher oxidation-level than the apical region. If the dye-reduction pic- 

 ture does exaggerate the change, the level may not actually be higher. On the other 

 hand, cytolysis and death by chemical agents may be relatively somewhat retarded 

 in the basal region by the greater thickness of the cell wall. If there is such retardation, 

 the real condition as regards these gradient patterns in the later blastula may be 

 intermediate between that indicated by differential death and differential dye reduc- 

 tion. But whatever the final conclusions as regards degree and stage, there seems to 

 be conclusive evidence that a change in condition does occur in the basal region, 

 beginning in the later blastula or at gastrulation in StrongyloccntroiHS, Dcndrastcr, and 

 Patina and probably at this stage or later in Arbacia, Eckinarachiiius, and Asterias. 

 The form of the Arbacia gastrula suggests that the change in condition in the basal 

 region, or at least in the entoderm, preceding gastrulation is not so great as in Strongy- 

 locentrotus and Dcndrastcr. The apicobasal axis of the Arbacia gastrula is relatively 

 shorter, its walls are thicker, and the archenteron remains thick walled during in- 

 vagination, most of its elongation and decrease in thickness occurring later than in the 

 other forms. 



